Sum Root to Leaf Numbers

Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.
An example is the root-to-leaf path 1->2->3 which represents the number 123.
Find the total sum of all root-to-leaf numbers.
For example,
1
/ \
2 3
The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.
Return the sum = 12 + 13 = 25.

题目大意：给定二叉树，根->叶的路径组成了一个数，返回所有数的和

题目难度：Medium

/**
 * Created by gzdaijie on 16/6/13
 */
public class Solution {
    public int sumNumbers(TreeNode root) {
        if (root == null) return 0;
        return helper(root, root.val);
    }

    private int helper(TreeNode root, int sum) {
        if (root.left == null && root.right == null) return sum;

        int left = 0, right = 0;
        if (root.left != null) left = helper(root.left, sum * 10 + root.left.val);
        if (root.right != null) right = helper(root.right, sum * 10 + root.right.val);
        return left + right;
    }
}